In one known type of pump, the inlet enters the pump chamber through a base thereof, and the outlet is also from the lower part of the pump chamber to provide clearance for the sweeping of the piston of an upwardly spring-biased cap plunger, with an outward seal against the pump chamber wall.
Valving of the inlet and outlet conventionally involves at least one ball valve. These are well-proven but require complicated assembly of a large number of parts, and are difficult to disassemble and clean thoroughly. The difficulty in cleaning is particularly troublesome when there is a need for hygienic pumping e.g. of food materials such as sauces.
U.S. Pat. No. 3,680,986 (Kutik & Gronemeyer) describes a pump which uses a one-piece valve in the form of a sleeve having a peripheral lip which forms an outlet valve and a central, flexibly-attached piece which forms an inlet valve. Both valves flex at the same level, however, and the sleeve serves no function other than for positive location and positioning of the valve body in deep recesses of the pump body. These will be very difficult to clean.